The present invention relates to a method for recovering a polishing agent which has been used in chemical-mechanical polishing of a semiconductor board or of a coating formed on top of a semiconductor board. The present invention also relates to a device for recovering such a polishing agent.
The surface of a semiconductor board and the surface of a coating formed on top of a semiconductor board are required to be as flat and planar as possible. Production of semiconductor integrated circuits often employs a photoresist patterning step, which uses exposure light to create a circuit pattern. In order to create more detailed patterns, a light having a shorter wavelength must be used. As a result of the short wavelength light, the allowable range for the depth of focus of the exposure can become less than 1 .mu.m. This short depth of focus in turn dictates that the exposure surface must be as flat as possible to accurately create the desired integrated circuit pattern.
However, in a semiconductor integrated circuit having multiple levels of wiring and three-dimensional wiring levels, the insulating surface between the wiring levels is usually not flat, due to the presence of the lower level wiring patterns. As a result, the exposed interlayer insulating film surface must also be flat. Even in a trench isolation configuration which completely planarizes the board surface by embedding an isolation oxide film in the board, the board and the embedding insulating film must be made as flat as possible.
In current semiconductor insulating films, Al, W, or Cu is applied by a reflow sputter method or a chemical vapor deposition method to form a metallic film on top of wire grooves. By chemical mechanical polishing of these metallic films, a flattened wiring having metal embedded in a wiring groove can be formed.
Chemical-mechanical polishing utilizes a polishing agent to planarize an insulating film or to planarize an embedded metallic thin film of Al, W, Cu, or the like. In this method, polishing is conducted with a slurry polishing agent interposed between a polishing member, such as a polishing pad or the like, and a semiconductor board. The semiconductor board itself or a coating, such as a silicon oxide film or a metal thin film or the like, may also be planarized by this method.
In chemical-mechanical polishing, fine silica particles are often used as the polishing agent, because silica particles exhibit good dispersion and uniformity in average particle diameter. Generally, fine silica particles are dispersed in a dispersion medium, such as water, and used as a silica suspension. The polishing agent is then generally discarded after a single use. Normally, 250-500 ml of polishing agent is used for chemical-mechanical polishing of a single board surface layer. However, as polishing costs have increased, the process costs of semiconductor production have also increased. A technique for retrieving and reusing used polishing agent has therefore become desirable as a means to lower semiconductor production costs.
When polishing with such a polishing agent, all of the following become mixed in with the polishing agent and are discharged as polishing waste water: the polishing debris chipped off from the polishing pad or chipped off from the thin film material which forms the semiconductor board surface coating layer (such as silicon oxide film); the extra fine particles, which are the destroyed silica particles of the polishing agent; and the large diameter polishing debris, which are aggregates of thin film material pieces and polishing particles.
Generally in a chemical-mechanical polishing process applied to an insulating layer, a pad conditioning step is performed prior to polishing. In this pad conditioning step, the top surface layer of the polishing pad is ground using a rotating file bearing an electrocoating of fine diamond particles. In this pad conditioning step or in a later pad washing step, distilled water is mixed with the polishing waste water. The polishing agent within the polishing waste water therefore usually becomes quite dilute. If polishing waste water is reused as a polishing agent without any processing, the following problems arise:
1) The large diameter particles in the polishing waste water, including the polishing debris and aggregates, can cause scratches in the board surface. Furthermore, the polishing strength may be reduced by the build-up of polishing debris.
2) The extra fine particles, which are generated from the breakdown of polishing particles, can result in board contamination. In particular, the adhesive strength between the board and the polishing agent particles relates to the surface tension. If the polishing agent particles becomes extremely fine, the surface area per unit of volume increases. The surface tension of the particles rises, and the extra fine particles adhere strongly to the board. As a result, these extremely fine particles cannot be removed from the board surface layer by washing, leading to board contamination.
3) When the polishing agent concentration becomes low, the polishing speed of the silicon board surface coating (silicon oxide film in this case) decreases undesirably.
Referring to FIG. 6, a schematic flow diagram of the polishing agent recovery method of the prior art as described in Japanese Laid-Open Patent Application 8-115892 is shown. A fine filtration device 101 is divided by a fine filtration membrane 101a into a concentrated solution compartment 101b and a filtrate compartment 101c. Similarly, an ultrafiltration device 102 is divided by an ultrafiltration membrane 102a into a concentrated solution compartment 102b and a filtrate compartment 102c. Fine filtration membrane 101a has a pore size around 500 nm, and ultrafiltration membrane 102a has a pore size around 10 nm.
The colloidal silica, which are the extra fine polishing agent particles, is recovered from the polishing solution as follows. A used polishing solution 105 is brought to a pre-processing solution container 103. Pressure is applied by a pump P1, and the solution is supplied to concentrated solution compartment 101b of fine filtration device 101. Fine filtration is conducted across fine filtration membrane 101a. Particles of colloidal silica and fine impurities below 500 nm in diameter, as well as the dispersing medium, are transmitted into filtrate compartment 101c. The filtrate is passed to a mid-process solution container 104 as a mid-process solution 106. Any large impurities greater than 500 nm in diameter remain in concentrated solution compartment 101b. The concentrated large impurity solution passes through a valve 107 and circulates back to pre-processing solution container 103. This operation is repeated, and when the concentration factor becomes 30-50 times, the concentrated large impurity solution is discharged as waste water 113a from a valve 108 to a waste water pathway 109.
Mid-process solution 106, which has had large impurities removed by the fine filtration step, is then supplied from mid-process solution container 104 to concentrated solution compartment 102b of ultrafiltration device 102 under pressure from a pump P2. Ultrafiltration is conducted across ultrafiltration membrane 102a. Any fine impurities less than 10 nm in diameter, as well as the dispersion medium, are transmitted to filtrate compartment 102c. The filtrate is discharged as waste water 113b to a waste water pathway 110. Colloidal silica particles having a diameter of 10-500 nm remain in the concentrated solution side. The concentrated solution recirculates to mid-process solution container 104 via a valve 111. This operation is repeated, and when the concentrated solution reaches a specified concentration (10-30% by weight), the concentrated solution containing the colloidal silica is recovered as a recovery solution 114 through a valve 112.
Referring to FIG. 7, an example which applies the recovery method of FIG. 6 for planarizing polishing of an interlayer insulation film is shown. A polishing element 115 has a polishing pad 115c on a rotating platform 115b which rotates inside a casing 115a. Polishing is conducted on a silicon board 116 held by a rotating board chuck 115d. Silicon board is polished by pushing silicon board 116 against polishing pad 115c while a polishing agent 117 and a pad washing solution 118 (water) are dripped onto polishing pad 115c. The irregularities on the surface of the silicon oxide interlayer insulation film (not shown) formed on the surface of silicon board 116 are thereby removed. The polishing of the silicon oxide interlayer insulation film proceeds by the chemical etching action of the silicon oxide and the mechanical friction of the polishing agent particles.
A used polishing solution 105 is discharged from polishing element 115. Processing debris from the interlayer insulation film, as well as silica particles and ammonium salts, are present in used polishing solution 105. In recovery preparation portion 120, recovery solution 114 is generated from used polishing solution 105 by recovery/concentrating part 121, as described in FIG. 6. Waste water 113, which includes processing debris and water, is discharged.
Recovery solution 114 is sampled from a primary sampling pipe 119a en route from a recovery solution pipe 119 to a solution mixing area 130. The silica density, pH, and ammonium salt concentrations are measured by a silica concentration measuring device 122, a pH meter 123, and a conductance meter 124, respectively. Each of the values is transmitted to a control device 125. To equilibrate the silica, density, pH value, and ammonium salt concentration of recovery solution 114 with a silica abrasion particle original solution 126, appropriate amounts of an ammonium salt solution 27 and distilled water 128 are added to recovery solution 114. The flow of recovery solution 114 inside recovery solution pipe 119 is adjusted using flow controllers 127a and 128a. All steps are automatically controlled by signals from control device 125.
Recovery solution 114 is then reused as a reusing solution 114a. Reusing solution 114a is also sampled by a secondary sampling pipe 119b. The silica concentration, pH, and conductance (ammonium salt concentration) of reusing solution 114a are also monitored using silica concentration meter 122, pH meter 123, and conductance meter 124, respectively. Controlling device 125 then microadjusts the flow rates of ammonium salt solution 127 and distilled water 128 to obtain the appropriate values. The components of recovery solution 114 are thereby made identical to those of silica abrasion particle original solution 126.
Reusing solution 114a is next brought to a solution mixing area 130. The flow of silica abrasion particle original solution 126 is adjusted by flow controller 126a in response to control signals from control device 125. Reusing solution 114a and silica abrasion particle original solution 126 are mixed at a predetermined ratio to form polishing agent 117. Polishing agent 117 is dripped onto polishing pad 115c on top of rotating platform 115b. The flow of polishing agent 117 can be adjusted by a flow adjuster 117a.
Though adequate recovery and reuse of polishing agent is possible with this prior art method, the following problems are inherent in this procedure.
First, fine filtration membrane 101a has a pore size intermediate between the particle size of the large impurities, which are to be removed, and the particle size of the polishing agent, which is to be recovered. When fine filtration membrane 101a is used to concentrate and remove polishing debris, blinding of fine filtration membrane 101a occurs rapidly. Frequent washing or replacement of fine filtration membrane 101a becomes necessary. Theoretically, using a fine filtration membrane with a pore size intermediate between the particle diameter of the polishing agent and polishing debris should separate the two, but when using this kind of fine filtration membrane, there is intense blinding. As a result, a cake layer forms on the membrane surface, and relatively small sized colloidal silica particles become trapped. Further blinding is generated, and the pressure difference across the membrane rises.
Second, there is no concentration sensor for the recovered solution in mid-process solution container 106. When the diluted polishing waste water is concentrated using the ultrafiltration membrane, the concentration of the polishing agent cannot be continuously measured. Therefore, it is not possible to control this concentration step automatically.
Third, there is no back-washing feature to control the blinding of fine filtration membrane 101a.
Fourth, there is only one system of fine filtration device 101. When changing blinded fine filtration membrane 101a, it becomes necessary to shut down the entire polishing agent recovery device.
Fifth, when circulating mid-process solution through ultrafiltration membrane 102a, pump P2 not only circulates the solution, but also increases the solution temperature. The rate of polishing increases with increased polishing solution temperature. As a result, increased polishing solution temperature lowers the stability and predictability of the chemical-mechanical polishing.
Sixth, there is no feature for periodically backwashing ultrafiltration membrane 102a of ultrafiltration device 102.
Seventh, only the polishing agent particles are recovered from the polishing waste water from the chemical mechanical polishing device. Water, one of the major components, is discharged. Therefore, this system does not completely recover the used polishing solution.
FIG. 7, in the prior art method shown in FIG. 7, there is no means for adjusting the pH of the recovered solution in recovery/concentration portion 121. As a result, the polishing agent particles aggregate within the pipes where recovery solution 114 is transported, further decreasing flow through the system.